The object of the invention is to provide a seismic exploration method and device permitting the emission into subsoil formations a series of seismic impulses irregularly distributed in time.
More particularly, the invention relates to a seismic prospecting method and device making it possible to locate the position of subsoil discontinuities, for example by exploiting the sign bit associated with the waves picked-up after the propagation thereof by a reception and acquisition system located at the surface or in a well following the emission of such a series of acoustic waves.
The method and the device according to the invention are particularly suitable for seismic prospecting operations on land around a well in the process of being drilled in order to restore the position of subsurface reflectors, if a drill bit producing seismic impulses as it moves forward in the crossed formations, such as a conventional rock bit, is used.
Seismic prospecting methods generally comprise emitting, into the formations to be prospected, seismic waves in the form of vibrations or impulses, receiving the waves which are propagated into the subsoil by a reception array comprising a plurality of sensors, recording the picked-up waves and processing recordings by a series of processing steps so as to improve the readability of seismic sections achieved from these recordings.
Methods through which the seismic waves are emitted by a source progressing with a drill bit and received by an array of sensors arranged at the surface are well-known through French Patent No. 1,584,951, U.S. Pat. Nos. 2,933,144 and 4,207,619 or patent application PCT WO85/05,696 for example. The waves can be produced by the drill bit itself in contact with the rocks to be drilled, or they can result from impacts applied to the drill bit during the progressing thereof, or else by a vibrating source interposed on the drill string.
A conventional process consists (through an operation known as cross-correlation) in cross-correlating each seismic trace obtained, as a whole, with the emitted signal, also as a whole, and then, by stacking, in combining it with other traces which have also been cross-correlated and which relate to common depth points (CDP).
U.S. Pat. Nos. 4,058,791, 4,346,461 or 4,543,632, for example, describe some applications of one general principle to seismic prospecting. According to this principle, the previous treatments are carried out from signals reduced to the signs therefo by clipping. The emitted signals are long-lasting vibrations whose frequency varies uniformly within a definite frequency spectrum, either uniformly or according to a pseudorandom code. The signals are emitted by a vibrating source arranged on the land surface or towed behind a ship within the framework of offshore seismic prospecting operations. Seismic methods implying selecting and treating only the sign bit are advantageous because the volume of data to be processed is substantially restricted, and the number of sensors and of seismic acquisition channels can therefore be increased. However, these prior methods are only fully efficient with particular vibrating sources.